1. Field of the Invention
The present invention relates to a method for pretreating the surface of aluminum and its alloys to prepare it to receive a coating to protect against corrosion or to improve adhesion of paint. In particular, the present invention relates to a surface preparation method that prepares the surface to provide an improved coating on aluminum and its alloys.
2. Description of Related Art
Aluminum and aluminum alloys are frequently used to form structures, such as for use in manufacturing aircraft, in which corrosion resistance is required or in which good paint adhesion is required. Aluminum has a natural oxide film which protects it from many corrosive influences. This natural oxide is, however, not sufficiently resistant to such highly corrosive environments as saltwater, nor is it a good base for paints. Improved films, which are both more corrosion resistant and suitable as a base for paints can generally be formed on the surface of aluminum either by anodizing or by chromate conversion. During the anodizing process, aluminum oxide is formed on the aluminum surface, and provides a very corrosion resistant surface which can be dyed or painted. However, anodizing has the disadvantage of high electrical resistance, higher cost, longer processing time, and the need to make direct electrical contact with the part. This latter requirement complicates processing considerably.
Chromate conversion coatings are formed by dipping the aluminum part in chromic acid, to provide a coating comprising chromium oxide(s) mixed with aluminum oxide. Chromate conversion coatings are corrosion resistant, provide a suitable base for paint, can be rapidly applied, self-heal when scratched, and are very cheap. Furthermore, chromate coatings are reasonably conductive and can be used in sealing surfaces for electromagnetic interference gaskets. The conductive characteristics provided by chromate conversion coating are not characteristic of anodized coatings nor of most protective coatings. Unfortunately, the hexavalent chrome used in producing these cheap, reliable, and useful coatings poses serious health hazards as well as significant disposal problems. Dermatitis and skin cancer have been associated with the mere handling of chromated aluminum parts. Severe damage to mucous membranes and skin lesions called "chrome sores" occur from exposure to the ever-present chrome mist in plating shops. Such health hazards to humans represent a major problem in the use of chrome for protecting aluminum. Thus, it would be desirable to replace the chromating process entirely.
Wash primers have been used in place of chromated conversion coatings. However, these usually contain phosphoric acid and chromates in order to promote adhesion of paint, powder coatings, and adhesive bonded joints.
A recently developed process which eliminates the use of chromium involves coating aluminum surfaces with a film of aluminum oxyhydroxide (pseudo-boehmite), as disclosed in U.S. Pat. No. 4,711,667, entitled "Corrosion Resistant Aluminum Coating". The process comprises, following degreasing, cleaning the aluminum-containing part in a cleaning solution which does not interfere with the bonding of the corrosion-resistant coating onto the surface of the part. Then, an aqueous solution comprising an alkali metal permanganate and a buffer compound is applied to the surface of the aluminum-containing part. This process yields a coating which is not as conductive as a chromate conversion coating, but is not, however, an insulator. In addition, its corrosion resistance is not as good as that produced by chromate conversion. This process is referred to herein as the "Sanchem process".
In another known method, aluminum has been treated with cerium chloride, CeCl.sub.3, to form a mixed cerium oxide/cermium hydroxide film on the surface, as described, for example, by B. R. W. Hinton et al, "Cerium Conversion Coatings for the Corrosion Protection of Aluminum", Materials Forum, Vol. 9, No. 3, pp. 162-173 (1986). In this process, a coating of cerium oxide/hydroxide is precipitated on the aluminum surface and provides a relatively high degree of corrosion resistance. Unfortunately, this process is slow, taking almost 200 hours. The speed of the process can be improved so that the coverage can occur in 2 to 3 minutes by cathodically polarizing the coupon. However, this leads to a less durable coating, and the process is inconvenient because it requires the use of electrodes.
Application U.S. Pat. No. 5,192,374, filed Sep. 27, 1991, and assigned to the same assignee as the present application, discloses an improved method of providing a protective coating on the surface of aluminum or aluminum alloys, comprising: removing contaminants from the surface; exposing the surface to water at 50.degree. to 100.degree. C. to form a porous boehmite coating on the surface; and exposing the boehmite-coated surface to an aqueous solution comprising a cerium salt and a metal nitrate at a temperature of 70.degree. to 100.degree. C. Oxides and nitrides of cerium are formed within the pores of the boehmite to provide the protective coating, which provides corrosion resistance and improved paint adhesion. This process is referred to herein as the "Hughes long form process".
Work continues to develop processes that reduce the number of steps or otherwise provide improved adhesion of paints and improved corrosion resistance of aluminum and its alloys for painting, powder coating, and adhesive bonding without the use of chromated conversion coatings or wash primers. For example, the prior art of using a chemical film for a paint base requires painting within an 8 hour period, otherwise, adhesion to base materials (aluminum alloys) is not reliable. In the event that the 8 hour period is exceeded, the chemical film must be removed and a primer used prior to painting with the standard painting process. For example, a wash primer per MIL-P-8514, "Coating Compound, Metal. Pretreatment; Resin Acid" is employed, followed by an epoxy primer and a polyurethane top coat.
However, there remains a need for a process that is not subject to any time constraints and does not require the use of a wash primer to activate the aluminum substrate. Such a process should be relatively short in duration and avoid the use of expensive chemicals, while providing substantially the same results as more expensive, longer processes.